1. Field of the Invention
The invention relates to piston and piston ring assemblies for internal combustion engines.
Pistons and associated piston rings are designed to reciprocate in a cylinder of an internal combustion engine. Combustion of fuel takes place above a crown of the piston and the pressure generated on combustion reciprocate the piston within the cylinder. The gap between the piston and the associated cylinder is commonly closed by three or more piston rings carried in respective grooves formed in a ring band extending around the crown of the piston. The rings exert an outward pressure against the associated cylinder or liner.
These piston rings function to provide a seal between the piston and the associated liner preventing the combustion products from passing down the gap between the piston and the associated cylinder. In addition, these rings control the film of oil which is provided on the cylinder wall to lubricate the reciprocation of the piston and rings and to aid the gas seal, so that excessive amounts of oil do not reach the combustion chamber while an adequate oil film is maintained.
The cylinder in which a piston reciprocates may be nominally truly cylindrical when the engine is cold, but as the engine reaches operating temperatures the shape of the cylinder will distort, in a way which depends on a number of factors, including the way in which the cylinder is formed and the mechanical loading of the cylinder in operation. The distortion of the cylinder is not constant along the axial length of the cylinder neither is it constant around the circumference of the cylinder. For this reason, it is not possible to design the piston rings to be an exact fit in a particular nominal cylinder size; they must be designed to conform as far as possible to the shape of the cylinder under all conditions and to the changes in shape along and around the cylinder. To achieve this, the piston rings are arranged to exert such an outward pressure on the associated cylinder, either due to inherent resilience of the spring or due to separate spring arrangements, that the shape of the rings tend to conform to changes in the shape of the cylinder.
2. Review of the Prior Art
In order to achieve this, and to provide a piston ring with an acceptable rate of wear, it has been proposed to form the rings of steel which, because of their bending characteristics, must be arranged to exert high pressures on the associated cylinder in order to achieve the required conformability.
The use of such steel rings has a number of consequences. The first is that, with present designs, experience has shown that it is necessary to have at least three rings; the two rings closest to the crown being conformed to provide seals preventing the passage of combustion products between the piston and the associated cylinder; so-called `compression` rings. The third, lowermost ring, is conformed to control the oil film on the associated cylinder; a so-called `oil control` ring. The presence of three steel rings all pressing against the associated cylinder with high outward pressures increases the frictional forces between the piston and the associated cylinder. Such frictional forces reduce the power output of the engine and also affect adversely the fuel consumption. The use of three rings also increases the piston height and weight and also increases the cost of the piston.